If ever we see the happy day when legislatures move to forbid public dollars for TV ads that benefit politicians in office, we’ll be leading the parade. But if Chris Christie is now to be politically hanged for his family’s eight-second appearance in a spot touting the Jersey Shore, we say: Investigate all governors.

For there’s little material difference between the $25 million New Jersey campaign that helped promote Christie and the $140 million “New York State Open for Business” campaign — which also used federal funding drawn from Sandy aid — to tout successes that promote Gov. Cuomo, even though he didn’t appear in them.

But news the feds are investigating Christie for his ads generates big headlines because it comes as Jersey’s governor is handling fallout from revelations that, contrary to what he told us earlier, his staff did close down lanes on the George Washington Bridge for crass political payback.

When the papers published the damning e-mail from his deputy chief of staff, Bridget Anne Kelly, to one of his appointees at the Port Authority, David Wildstein, Christie admitted he’d been proven wrong. He fired Kelly for having lied to him and sacked his two-time campaign adviser, Bill Stepien, who knew what happened and hadn’t stepped up. Two appointees at the Port Authority had already resigned and face their own problems. In the week since, we’ve seen no evidence to contradict Christie’s account of what he knew about the lane closures and when.

So by all means, let Christie answer for the breach of trust committed by people he appointed. But the 24/7 “Christie scandal” coverage we now see suggests that what’s really driving some of these headlines isn’t about TV ads or what happened on the George Washington Bridge last September. It’s about ensuring that a leading Republican contender for president won’t be on the ballot come November 2016.